1. Field of the Invention
Embodiments of the invention generally relate to the closed loop control of a process variable and more specifically to closed loop control of liquid delivery for wet processing of substrates in the fabrication of electronic devices.
2. Description of the Related Art
Wet processing of substrates in the fabrication of electronic devices, particularly in electrochemical plating (ECP) and chemical/mechanical polish (CMP) applications, often require precisely controlled chemical deliveries for very short process steps.
Typically, when a substrate is treated in a process chamber, a series of treatments will take place sequentially in the same chamber. This series of treatments, or process steps, is generally referred to as a process recipe. Process recipes are application specific and, therefore, vary depending on which electronic device manufacturer is operating the chamber, what device is being fabricated on the substrate, and sometimes which particular film of the device is currently on the surface of the substrate (i.e., metal 1 vs. metal 2, etc.). In the case of the chemical delivery steps in a wet wafer processing chamber, the process steps in a process recipe are often as short as 10–15 seconds. Under these conditions, accurate and repeatable control of liquid chemical delivery is difficult, and failure to provide such control directly affects substrate quality and device yield. Standard control methods for this application include open loop control and closed loop control.
A simple method of controlling liquid delivery with open loop control uses a fixed orifice with constant pressure. This method controls flow with an adjustable, fixed orifice, for example a needle valve, and a pressure regulator, which maintains a constant fluid pressure upstream of the needle valve and therefore maintains a constant flow rate. For a number of reasons such control has proven inadequate for chemical delivery to a wet processing chamber for the processing of a semiconductor substrate. Chronic problems with this method include: drift from setpoint of both the fixed orifice and the pressure regulator, poor repeatability chamber-to-chamber and substrate-to-substrate, cross-talk between chambers (i.e., actual flow rate to one chamber is affected by whether there is flow to other chambers at the same time), difficulty in monitoring flows without introducing additional sensors, and an inability to incorporate alarms when the flow rate is too far from the target setpoint. Additionally, any changes to the liquid delivery system that affect liquid flow cannot be compensated for with open loop control. Examples of changes include: liquid tubing re-routed or kinked, liquid tubing replaced with tubing of different length or inner diameter, liquid delivery nozzles changed, and flow regulator performance altered due to mechanical wear. When such changes occur, re-calibration is required. Therefore, in order to achieve stable and repeatable DI water and chemical flow to process chambers it is necessary to have a closed loop flow control system.
Closed loop control is based on continuously modifying the desired process variable based on the measured value of the process variable and the setpoint for the process variable. This involves measuring the desired process variable with an appropriate sensor, generating a signal proportional to the measurement, sending this signal to a computer, processing the signal in the computer (using a control algorithm that determines what adjustment needs to be made to the process variable in order for the process variable to be closer to the target setpoint value), and outputting a signal to a control device that is proportional to the desired correction. The amount of correction is a function not only of how much the process variable needs to be adjusted, but also other factors: sampling rate, response times of the control device and sensor, and physical factors such as valve sizes and sensor locations. The process variable is then measured again and the process is repeated, typically at a relatively high frequency, for example every 50 milliseconds. Closed loop control allows for stable and repeatable control of a process variable. The state-of-the-art method used for closed loop control is PID (proportional, integral, derivative) control. With the correct tuning of PID control parameters, this algorithm can provide process variable control over a wide range of situations.
In the case of liquid delivery in a wet substrate processing chamber, a typical closed loop control system might consist of an ultrasonic flow meter or vortex flow meter (FM) as a flow sensor, a pneumatically controlled pressure regulator as a flow regulator, a dedicated computer processing the input signal from the FM with a PID algorithm, and a voltage-to-pressure transducer to convert the output signal voltage from the computer to a proportional pneumatic pressure to operate the control device. For situations involving continuous control of the flow over relatively long periods of time, for example minutes or hours, this system can work well. However, for short process recipe steps that require controlling the flow from full off to a given setpoint in a short time, PID control is not very effective. This is because when tuning a PID control loop, there is a direct trade off between quick response time and minimal overshoot past the setpoint.
In the case of liquid chemical delivery for the treatment of semiconductor substrates, it is very important to avoid overshoot of the flow rate setpoint. This is because overshoot can result in splashing of chemical in the process chamber, which can cause serious defects on the surface of the substrate. Also, for some semiconductor applications, the liquid chemicals must be delivered at precise flow rates, otherwise, serious defects can occur.
For a sudden change in setpoint that only lasts a short time, PID control will still be stabilizing the flow (i.e., hunting above and below the new setpoint) during some or the entire process recipe step. This results in chronic and often serious variations in the process each time the recipe is run. Another problem with PID control in some situations is the fact that it is a calculation intensive method for determining how much the process variable needs to be corrected. This is because no matter how close the process variable is to the target setpoint, the PID algorithm will continue to calculate a correcting adjustment. Often this is not important, but if the computer performing these calculations is shared by a large number of sensors and control devices, for example as with the system controller for a wafer processing system, the response time of the entire system can be slowed, dramatically affecting the ability of the wafer processing system to function. Additionally, if the control device in such a control loop is subject to a significant amount of hysteresis, more sophisticated algorithms need to be incorporated into the PID calculation to offset this effect. This makes the control process even more calculation intensive.
Another issue encountered when controlling fluid flow or other process variables is drift. Drift is caused by long-term systemic changes that increasingly affect the control of a process variable and/or the calibration of sensors or control devices. Factors that contribute to the drift of sensors, control devices, and other mechanical components include normal wear and tear, accumulated contamination, and loss of calibration. Drift can be difficult to detect since it generally takes place over a long period of time (weeks or months) and a conventional closed loop control will compensate until it can no longer keep the process variable at setpoint. An example of this is the mass flow controller (MFC) used in semiconductor manufacturing applications, which generally operates as a self-contained PID closed loop controller that supplies process gas at a specific flow rate. Typically, a central system controller will communicate a desired setpoint to the MFC for a particular recipe and the MFC's internal closed loop control will maintain gas flow at that setpoint. If drift of the gas delivery system being controlled occurs over time (for example due to a loaded filter), the MFC will continue to compensate to keep the process variable at setpoint. It will only send an alarm to a central system controller once it is fully open and still cannot control the gas flow to the setpoint. For most semiconductor processes, this is too late to avoid compromising substrate quality/yield. To prevent this, conservative preventive maintenance (PM) schedules are typically followed for semiconductor manufacturing equipment, for example early filter replacement. This leads to greater system downtime and inefficient use of spare parts and labor.
Therefore, there is a need for a method of closed loop control that:                provides the fastest possible response to sudden changes in process variable setpoint while minimizing or eliminating overshoot of the process variable;        allows the early detection of system drift affecting the process variable;        compensates for hysteresis in the control device of the control system; and        performs the above functions with significantly fewer calculations than demanded by the PID method.        